In recent years, solid electrolyte fuel cells are required to be operated under a low humidity environment, primarily in their use for automobiles. Therefore, a material which exhibits high proton conductivity in a low humidity environment is desired for a proton-conductive polymer contained in a polymer electrolyte membrane for a membrane/electrode assembly.
In actual driving of an automobile having a polymer electrolyte fuel cell mounted, the polymer electrolyte membrane will be exposed to various humidity environments. As a test to simulate such a situation, a cycle test (hereinafter referred to as a moistening/drying cycle test) has been proposed wherein the polymer electrolyte membrane is repeatedly exposed to each of a dried environment with a relative humidity of at most 25% and a moistening environment with a relative humidity of 100% (Non-Patent Document 1). In such a moistening/drying cycle test, the polymer electrolyte membrane swells in the moistening environment and shrinks in the dried environment and thus undergoes swelling and shrinkage repeatedly along with moistening and drying cycles and thus undergoes a dimensional change especially in a planar direction. Therefore, by the moistening/drying cycle test, it is possible to evaluate the mechanical durability of the membrane/electrode assembly in an environment where moistening and drying are repeated.
A polymer electrolyte fuel cell is constructed, for example, by stacking a plurality of membrane/electrode assemblies each having electrodes (a cathode (an air electrode) and an anode (a fuel electrode)) disposed on both sides of a polymer electrolyte membrane, via an electrically conductive separator having a gas channel formed therein. Each electrode is composed of a catalyst layer in contact with a polymer electrolyte membrane and a porous gas diffusion layer disposed outside of the catalyst layer.
It has been always required that a polymer electrolyte fuel cell has a high power generation performance. Accordingly, a material capable of exhibiting a high proton conductivity is required for a proton-conductive polymer contained in the polymer electrolyte membrane and catalyst layer of the membrane/electrode assembly.
In order to improve the proton conductivity of the proton-conductive polymer, the ion exchange capacity may be increased. However, if the ion exchange capacity is increased, the water content of the proton-conductive polymer will increase. If the water content of the proton-conductive polymer to be used for the catalyst layer becomes too high, the proton-conductive polymer will swell and clog void spaces in the catalyst layer, thus leading to a problem of a so-called flooding phenomenon. If the flooding phenomenon takes place, the diffusion of the gas supplied to the catalyst layer decreases, whereby the power generation performance of the polymer electrolyte fuel cell will be substantially deteriorated.
To cope with the above problem, a membrane/electrode assembly of the following (1) and a polymer electrolyte fuel cell of the following (2) have been proposed.
(1) A membrane/electrode assembly for polymer electrolyte fuel cells, which has a catalyst layer containing at least an electrode catalyst and a proton-conductive polymer on each side of a polymer electrolyte membrane, wherein at least one catalyst layer is made of a plurality of layers so that it has a proton-conductive polymer layer having EW (equivalent weight of proton-conductive exchange groups) different from the polymer electrolyte membrane at the interface where at least one of the catalyst layers is in contact with the polymer electrolyte membrane, and it has a gas diffusion layer containing at least an electrically conductive porous material and a fluoropolymer, on the side of the catalyst layer opposite to the side in contact with the polymer electrolyte membrane (Patent Document 1).
(2) A polymer electrolyte fuel cell comprising an anode, a cathode and a polymer electrolyte membrane disposed between the anode and the cathode, and designed to supply an anode reaction gas to the above anode and supply a cathode reaction gas to the above cathode thereby to generate an electric power by an electrochemical reaction, wherein the cathode is provided with a gas diffusion layer and a plurality of catalyst layers disposed between the gas diffusion layer and the polymer electrolyte membrane, and the ion exchange capacity X (meq/g dry resin) of the proton-conductive polymer contained in the innermost catalyst layer in contact with the polymer electrolyte membrane and the ion exchange capacity Y (meq/g dry resin) of the proton-conductive polymer contained in the outermost catalyst layer in contact with the gas diffusion layer satisfy the following conditions simultaneously (Patent Document 2):0.88≦X≦1.500.70≦Y≦1.320.18≦(X−Y)≦0.70
Further, to cope with the above problem, a polymer electrolyte fuel cell is, for example, proposed which is designed so that on the surface of a cathode on the side opposite to the side in contact with a polymer electrolyte membrane, a gas containing oxygen is permitted to flow in parallel with said surface, and in the cathode, the ion exchange capacity of a proton-conductive polymer at a portion in contact with the upstream of the gas flow is “higher” than the ion exchange capacity of a proton-conductive polymer at a portion in contact with the downstream of the gas flow (Patent Document 3).
Non-Patent Document 1: Yeh-Hung Lai, Cortney K. Mittelsteadt, Craig S. Gittleman, David A. Dillard, “VISCOELASTIC STRESS MODEL AND MECHANCIALS CHARACTERIZATION OF PERFLUOROSULFONIC ACID (PFSA) POLYMER ELECTROLYTE MEBRANES”, Proceedings of FUELCELL 2005, Third International Conference on Fuel Cell Science, Engineering and Technology, FUELCELL 2005, (2005), 74120
Patent Document 1: JP-A-11-288727
Patent Document 2: JP-A-2001-338654
Patent Document 3: JP-A-2001-196068